In the US, university functions like a rite of passage into adulthood.

Which i think is precisely the reason it shouldn't be like that across other countries. This is very derived from anglo-culture but historically university has been for all ages groups; public education in Europe is the reason people started doing it later, but realistically some people could be doing it earlier and some people shouldn't be doing it until later in life when they're more sure of what they want. To me the tragedy of public education is that it holds smart and hardworking kids back until the less motivated ones catch up with them.

The brain i suppose does keeps growing until say midlife, 30s, or 25 (most things keep developing until 25) - this is something doctors believe when I talk to them. But as my above paragraph alludes: some people reach mental maturity faster. I would rather we look at uni as a scholarly pursuit again and detach the societal pressure to engage in hedonism. It (university) would benefit as a system engaged-in by all age groups.